In recent years, communication traffic upgrade has been swiftly made to exchange huge amounts of data at high speed by using light in the information communication area. Currently, long-haul fiber optic networks, such as backbone, metro, and access networks, spread over a relatively long distance that reaches a few kilometers or more. Further, existing signal lines are being replaced with fiber-optic lines in order to process huge amount of data without delay even for a very short distance between transmission devices (several meters to several hundred meters) or between devices (several centimeters to several ten centimeters).
High frequency signals, such as Ethernet are inputted to a line card through an optic fiber from the outside for fiber-optic wiring in a transmission device, for example, such as a router/switch device. A few line cards are provided for a single back plane, so that signals, each of which is inputted to each line card, are gathered into a switch card and then processed by an LSI included in the switch card, and then outputted to the respective line cards through the back plane. For the conventional devices, signals having a data rate of more than 300 Gbit/s are gathered from the respective line cards through the back plane to the switch card. The transmission of these signals through conventional electrical lines requires each electrical line to be responsible for data rate of 1 to 3 Gbit/s, and thus 100 or more electrical lines are needed.
Subsequently, a waveform shaping circuit or measures against reflection or cross talk between the lines are needed for such high frequency lines. As the system is developed to process huge amount of information, for example, of more than several terabits/sec, problems with the number of lines or measures against cross talk become more serious in the conventional electrical lines. To resolve these problems, there is highlighted a method of implementing signal transmission lines between the boards, for example, such as between the line card and the back plane, and between the back plane and the switch card with optical fibers. This method allows for low-loss transmission of high frequency signals whose data rate reaches more than 10 Gbps. Therefore, the number of lines is reduced and the above measures are not necessary even for high frequency signals, so that this method is considered promising.
Higher-density optical wiring and substrate mounting technologies including easy production methods are necessary to implement these high-capacity optical interconnection circuits. Patent Document 1 discloses an example of mounting a high-density optical connection of a multi-layer optical waveguide array and a photoelectric conversion element array with respect to high-density wiring, which is shown in FIG. 10. In this example, the optical wiring layers 101A and 101B, such as a plurality of optical waveguide arrays arranged two-dimensionally are stacked one above the other in the thickness direction of the substrate and connected to the surface light emitting (receiving)-type photoelectric conversion element array 100 mounted on a substrate surface, and this allows for high-density wiring with a reduced mounting area. Further, the mirror parts 106 for turning the optical path of light propagating through the optical waveguide array in the direction perpendicular to the substrate may be formed by bringing together the plurality of cores through a cutting process while the ends of the optical waveguide arrays 101A and 101B are arranged in the same column, and thus fabrication process is simplified.
Patent Document 2 discloses another example of a high-density optical connection between an optical wiring layer and a photoelectric conversion element array, which is shown in FIG. 11A-C. In this example, the light inlet/outlet ends of the adjacent optical waveguide cores 110, which may be in plural, are arranged to deviate from the waveguide direction of the light. Similarly, the photoelectric conversion element arrays 112, which are positioned to correspond to the light inlet/outlet ends of the cores, are also arranged to deviate from the waveguide direction, and optically connect to the optical waveguide cores 110. This may reduce the influence of cross talk between the adjacent cores or adjacent photoelectric conversion elements and further improve the integration density of the optical wiring and the photoelectric conversion element.
Patent document 1: Japanese Patent Application Publication No. 2003-114365.
Patent document 2: Japanese Patent Application Publication No. 2005-340545.